1. Field of the Invention
The present invention applies to the field of communications systems and, in particular, to training sequences and training signals.
2. Description of the Prior Art
Some communications systems, such as cellular voice and data communications systems, have several base stations in different locations available for use by mobile or fixed user terminals, such as cellular telephones or wireless web devices. Each base station communicates with a user terminal using a communications channel. For example, a communications channel may consist of a time slot in a TDMA (Time Division Multiple Access) frame on a physical carrier frequency. A TDMA frame may contain, for example, three uplink receive time slots followed by three downlink transmit time slots, or vice-versa. The time slots may be used to transmit communication bursts, or they may be delineated on a continuous signal.
A physical carrier frequency may be a 625 kHz band around a central frequency, such as 800 MHz or 1.9 GHz. Thus, a base station transmits to a given user terminal, for example, on the second transmit and receive time slots on this carrier frequency in a given frame. Furthermore, the communications channel may be organized using common duplexing techniques, such as FDD (Frequency Division Duplex) and TDD (Time Division Duplex), and common multiple access techniques such as FDMA (Frequency Division Multiple Access) and CDMA (Code Division Multiple Access). The channel may further be organized according to a hopping function indicating alternating resources over time.
The communications channel can be used for sending signals that communicate information, such as user data and control data. The communications channel may also be used for sending signals that are known at the receiver. Such signals are known as training or pilot signals. A training signal can be generated in many ways, such as sending a known symbol sequence, typically called a training sequence. In portions of the description below, the terms training signal and training sequence may be used interchangeably.
Training signals and training sequences can be used for measuring channel parameters and characteristics, such as SNR (signal to noise ratio), spatial parameters, timing, and frequency offset. They can also be used for synchronizing symbols and frames, calibrating transceivers and equalizers, and calculating spatial and temporal filter weights. One reason training sequences are useful, is that the received signal can be compared with the known sent signal, e.g., the known training sequence. Training sequences are used for “training,” which generally means performing some operation including comparing a received signal to a known reference signal. Thus, the above example uses of training signals and training sequences all constitute training.
A well-designed family of training sequences may have some useful or desirable properties. For example, one useful property that a family of training sequences can have is that two different training sequences from the family are as different as possible for all or most shifts. Another useful property that a family of training sequences can have is that a delayed, or shifted, training sequence in the family should be different from the same training sequence without shift. The first property is enhanced if the absolute value of the cross-correlations of any two training sequences is kept small. The second property is enhanced if the out-of-phase auto-correlations of the training sequences are kept small. Different systems may be enhanced by using different tradeoffs between various properties of training sequences, including the size of the sequences and the correlation properties described above.